The Orthogonal Frequency Division Multiplexing (OFDM) is a high speed transmission technology, and a basic principle of the technology is to transform high speed serial data into multiple paths of relatively low speed parallel data and modulate different carriers. The parallel transmission system greatly extends a pulse width of a symbol, and improves a performance of anti-multipath fading. The Polarization Division Multiplexing (PDM) technology may reduce a baud rate of the system by a half, relaxes a bandwidth requirement on high speed electronic devices, allows smooth upgrade from an existing 10 Gb/s commercial optical transmission network, and is a technology of great potential. The OFDM in combination with the PDM technology is a high speed optical transmission development trend worth being focused on in the future.
In a PDM OFDM system, the OFDM technology is adopted; a signal period becomes longer; and a Cyclic Prefix (CP) is introduced. Therefore, the influence of the inter-symbol interference brought by the dispersion may be constrained, but meanwhile, a channel also brings distortion to a signal. For example, a sent signal of a kth sub-channel is Stk=Ark·jj·φtk, in which Atk is an amplitude of the signal, and φtk is a phase of the signal. A frequency domain response of the channel in the sub-channel is Hk=AHk·ej·φHk, in which AHk is an amplitude-frequency response of the channel, and φHk is a phase-frequency response of the channel. Then, a received signal is: Srk=Hk·Stk=AHk·Atk·ej·(φHk+φtk), and it can be known that, an amplitude of the received signal is Ark=AHk·Atk, and a phase thereof is φrk=φHk+φtk. It can be known that, both the amplitude and the phase of the received signal are influenced by the channel, and signal distortion is caused, so electronic equalization processing needs to be performed on the channel, so as to eliminate the influence on the signal brought by the channel.
In another aspect, when a light beam is transmitted in an optical fiber, random birefringence occurs, so two polarization states such as an X polarization state and a Y polarization state of the sent signal in the transmission procedure continuously rotate. However, at a receiving end, channel characteristics are unknown, and two polarization states such as an X′ polarization state and a Y′ polarization state of the received signal do not track rotation of the X polarization state and the Y polarization state in real time, so a cross-talk occurs between two paths of received signals. A mathematical model of transmission in the channel is represented with a matrix as follows:
      [                                        X            r            k                                                            Y            r            k                                ]    =            [                                                  H              xx              k                                                          H              xy              k                                                                          H              yx              k                                                          H              yy              k                                          ]        ⁡          [                                                  X              t              k                                                                          Y              t              k                                          ]      
Xrk=Hxxk·Xtk+Hxyk·Ytk and Yrk=Hyxk·Xtk+Hyyk·Ytk are obtained.
[•]k represents the kth sub-channel; Xtk is the sent signal in the X polarization state; Xrk is the received signal in the X′ polarization state; Ytk is the sent signal in the Y polarization state; Yrk is the received signal in the Y′ polarization state; Hxxk represents a direct channel of the sent signal in the X polarization state; Hxyk represents a cross-talk channel of the sent signal in the Y polarization state for the sent signal in the X polarization state; Hyxk represents a cross-talk channel of the sent signal in the X polarization state for the sent signal in the Y polarization state; and Hyyk represents a direct channel of the sent signal in the Y polarization state. It can be known from the formula that, Xrk and Yrk respectively include different components of the X polarization state and the Y polarization state, and the cross-talk brings extremely serious damages to the system performance, so electronic depolarization processing needs to be performed on the two received signals.
The prior art does not have a relevant solution for the problem that the electronic equalization and the electronic depolarization are required to be performed on the PDM OFDM system. An existing electronic depolarization solution is mainly for a single-carrier system, and the electronic depolarization in the prior, art is performed in a time domain, that is, the electronic depolarization is directly performed on a time domain signal, so the complexity of the computation and the hardware is relatively high.